Osteoporosis is a leading cause of mortality and morbidity in aging women. The mechanisms of phosphate and calcium balance were studied as part of a major effort to understand the pathophysiological bases of senile osteoporosis and osteomalacia. Notable scientific achievements include the findings that (1) the active form of Vitamin D, 1,25-dihydroxy vitamin D3, incubated in vitro with isolated kidney cells stimulates phosphate uptake, this increase being blocked by inhibitors of RNA and protein synthesis; (2) in vitamin D deficiency, the resultant secondary hyperparathyroidism causes blunted responses to PTH, at a site that generates cAMP and at a site distal to the activation of adenylate cyclase; (3) vitamin D3 modifies membrane structure and function by liponomic regulation; (4) renal phosphate reabsorption is regulated by the serum phosphate level by a PTH-independent mechanism; and (5) PTH increases sodium-calcium exchange, a finding that may explain how the hormone acts to enhance calcium transport.